The present invention relates to techniques for drilling oil, gas, water, or geothermal wells or the like. More precisely, the invention relates to cementing compositions which are particularly suitable for cementing zones which are subjected to extreme dynamic stresses.
In general, a well which is over a few hundred meters deep is cased and the annular space between the subterranean formation and the casing is cemented over all or part of its depth. Cementing essentially prevents the exchange of fluid between the different layers of formation traversed by the hole and controls the ingress of fluid into the well, and in particular limits the ingress of water. In production zones, the casingxe2x80x94and the cement and the formationxe2x80x94is perforated over a height of several centimeters.
The cement placed in the annular space-of an oil well is subjected to a number of stresses throughout the lifetime of the well. The pressure inside a casing can increase or decrease because the fluid which fills it can change or because a supplemental pressure is applied to the well, for example when the drilling fluid is replaced by a completion fluid, or during a stimulation operation. A change in temperature also creates stress in the cement, at least during the transition period preceding temperature equilibration between the steel and the cement. In the majority of the above cases, the stress event is sufficiently slow for it to be treated as a static event.
However, the cement is subject to other stresses which are dynamic in nature, either because they are produced over a very short period or because they are either periodic or repetitive in nature. Perforations create an over-pressure of several hundred bars inside a well which is dissipated in the form of a shock wave. Further, perforations create a shock when the projectile penetrates the cement and that shock subjects the zone surrounding the hole to large forces over a depth of several meters.
A further event, which is now routine in oil well operations and which creates dynamic stresses in the cement, is the opening of a window in a casing which is already cemented to create a multi-branch lateral well. Milling the steel over a depth of several meters followed by drilling a lateral well subjects the cement to shocks and vibrations which frequently damage it irreparably.
The present invention aims to provide novel formulations, in particular for cementing regions of oil wells or the like which are subjected to extreme dynamic stresses.
In an article presented at the SPE (Society of Petroleum Engineers) annual technical conference and exhibition of 1997, Marc Thiercelin et al. (SPE 38598, Oct. 5-8, 1997)xe2x80x94and French patent application FR-A-97 11821 of Sep. 23rd, 1997, demonstrated that the risk of rupture of a cement sleeve depends on the thermoelastic properties of the casing, the cement and the formation surrounding the well. A detailed analysis of the mechanisms leading to rupture of the cement sleeve has shown that the risk of rupture of a cement sleeve following an increase in pressure and/or temperature in the well is directly linked to the tensile strength of the cement and is attenuated when the ratio between the tensile strength RT of the cement and its Young""s modulus E is increased.
Young""s modulus is known to characterize the flexibility of a material. To increase the RT/E ratio, it is advantageous to select materials with a low Young""s modulus, in other words to select very flexible materials.
One known means for increasing the flexibility of a hardened cement is to reduce the density of the slurry by extending it with water. However, that leads to a degradation in the stability of the slurry, in particular with separation of the solid and liquid phases. Such phenomena can, of course, be controlled in part by adding materials such as sodium silicate, but the permeability of the hardened cement is nevertheless very high, which means that it cannot-fulfill its primary function of isolating zones to prevent fluid migration, or at least it cannot guarantee its long-term isolation. Further, lightened cements have lower strength, in particular lower shock resistance, which constitutes a clear handicap for cements intended for use in zones which are subjected to extreme mechanical stresses such as perforation zones.
In the building field, incorporating particles of rubber into a concrete is known to result in better resilience, durability and elasticity (see, for example, A. B. Sinouci, Rubber-Tire Particles as Concrete Aggregate, Journal of Materials in Civil Engineering, 5, 4, 478-497 (1993)]. Concretes which include rubber particles in their formulation can be used, for example, in highway construction to absorb shocks, in anti-noise walls as a sound insulator and also in constructing buildings to absorb seismic waves during earthquakes. In such applications, the mechanical properties in particular are improved.
In the field of oil well cementing, it is also known [Well Cementing 1990, E. B. Nelson, Schlumberger Educational Services] that adding ground rubber particles (grain size in the range 4-20 mesh) can improve the impact strength and bending strength. Such an improvement in mechanical properties has also been indicated in Russian patents SU-1384724 and SU-1323699. More recently, United States patent U.S. Pat. No. 5,779,787 has proposed the use of particles derived from recycled automobile tires with grain size in the range 10/20 or 20/30 mesh, to improve the mechanical properties of hardened cements, in particular to improve their elasticity and ductility.
The present invention aims to provide oil well cements reinforced with flexible particles, of low compressibility, with low density and with an average size not exceeding 500 xcexcm.
The term xe2x80x9cflexible particlesxe2x80x9d means particles made of a material having a Young""s modulus of less than 5000 MPa, preferably less than 3000 MPa, more preferably less than 2000 MPa. The elasticity of the materials selected for these flexible particles is thus at least four times greater than that of cement and more than thirteen times that of the silica usually used as an additive in oil well cements.
The flexible particles added to the cementing compositions of the invention are also remarkable because of their low compressibility and are characterized by a Poisson ratio of over 0.3.
In order to lighten the slurry, it is also important for the density of the flexible particles to be less than 1.5 g/cm3, preferably less than 1.2 g/cm3, more preferably less than 1 g/cm 3. Preferably, this low density is intrinsic in the choice of the constituent materials and not by dint of high porosity or hollow particles. Preferably again, materials of low porosity are used.
Further, the particles must be insoluble in an aqueous medium which may be saline, and must be capable of resisting a hot basic medium, since the pH of a cementing slurry is generally close to 13 and the temperature in a well is routinely over 100xc2x0 C.
Regarding particle size, essentially isotropic particles are preferred. Spherical or near spherical particles may be synthesized directly, but usually the particles are obtained by grinding, in particular cryo-grinding. The average particle size is generally in the range 80 xcexcm to 500 xcexcm, preferably in the range 100 xcexcm to 400 xcexcm. Particles which are too fine, or on the other hand too coarse, are difficult to incorporate into the mixture or result in pasty""slurries which are unsuitable for use in an oil well.
Particular examples of materials which satisfy the various criteria cited above are thermoplastics (polyamide, polypropylene, polyethylene, . . . ) or other polymers such as styrene divinylbenzene or styrene butadiene (SBR). Recycled products are generally not preferred because of the variability in supply sources and in physico-chemical properties.
In addition to the flexible particles of the invention, the cementing compositions of the invention comprise a hydraulic binder, in general based on Portland cement and water. Depending on the specifications regarding the conditions for use, the cementing compositions can also be optimized by adding additives common to the majority of cementing compositions, such as suspension agents, dispersing agents, anti-foaming agents, expansion agents (for example magnesium oxide), fine particles, fluid loss control agents, gas migration control agents, retarders or setting accelerators. Thus the systems are either bimodal in type, the solid fraction of the slurry being constituted by a mixture of cement and flexible particles, or they can comprise three (trimodal), or more types of solid constituents, the solid mixture comprising fine micronic particles and possibly submicronic particles in addition to the cement and flexible particles.
The volume of flexible particles represents between 5% and 40% of the total volume of the cementing slurry, preferably between 10% and 35%, and preferably again, between 15% and 30% of the total slurry volume.
The formulations of the invention are preferably based on Portland cements in, classes A, B, C, G and H as defined in Section 10 of the American Petroleum Institute""s (API) standards. Classes G and H Portland cements are particularly preferred but other cements which are known in this art can also be used to advantage. For low-temperature applications, aluminous cements and Portland/plaster mixtures (deepwater wells, for example) or cement/silica mixtures (for wells where the temperature exceeds 120xc2x0 C., for example) can be used.
The water used to constitute the slurry is preferably water with a low mineral content such as tap water. Other types of water, such as seawater, can possibly be used but this is generally not preferable.
These particles with low density with respect to the cement can reduce the density of the slurry and result in lower permeability and better impact resistance. It also affects the flexibility of the system, since adding flexible particles produces cements with a lower Young""s modulus.
The compositions comprising flexible particles of the invention have remarkable mechanical properties which render them particularly suitable for cementing in areas of an oil well which are subjected to extreme stresses, such as perforation zones, junctions for branches of a lateral well or plug formation.